Slate

Physical Properties

Hardness: 2.5-4 (Mohs); Color: Dark grey to charcoal black; Luster: Dull to satiny; Crystal Structure: Microcrystalline; Cleavage: Excellent slaty cleavage; Specific Gravity: 2.7-2.8.

Formation & Geological History

Formed from the low-grade metamorphism of shale or mudstone under relatively low pressure and temperature conditions. It is common in folded mountain belts and dates back across various geological eras from the Paleozoic to the Cenozoic.

Uses & Applications

Widely used for roofing tiles, flooring, laboratory bench tops, billiard table beds, and as a natural material for walkways or stone walls.

Geological Facts

Slate naturally splits into thin, flat sheets due to its slaty cleavage, making it the perfect material for early writing boards known as 'slates.' It is highly durable and fireproof.

Field Identification & Locations

Identify by its ability to split into thin sheets and its dull, dark appearance. It's commonly found in areas such as Wales, Pennsylvania, and the Appalachian mountains. Field identification involves tapping it to hear a 'ring' versus the 'thud' of shale.

Identified on: 5/3/2026

Mode: Standard